To eliminate the defects of conventional structure made of steel-frame, etc., such as heavy weight and high bulkiness.

A part of a plate made of a soft plastic material and having a plane cross-section and an angle cross-section is formed to have a thin thickness to form an easily bendable part 1 which can be bent to a prescribed angle at the part 2. A long pipe having a pentagonal lug 8 can be formed by inserting a protrusion 5 of a joint plate 4 into a side hole 6 and fixing the joint plate 4 to the angle plate 3 with the holes 7 at both ends. A greenhouse structure is constructed by inserting the assembled pipes 10 into the holes of side face 9 and attaching glass plates. A plastic frame or structure of a livestock barn, etc., can be formed by providing the plane plate 11 with a bent part 1, pressing a protrusion 13, attaching a pin 14 and fixing with a wedge pin and the joint plate to form a quadrangular pipe. The plastic structure can be used as a reinforcing structure 17 for a wooden building by attaching square timbers to the upper and lower parts. The item 18 is a greenhouse having double-layer structure. The item 19 gives a structure having a wide space by forming to a semicircular form, bending at the groove part 20, fitting a joint plate to form a triangular pipe and connecting with the joint plate.